Crude oil from which desired gaseous and liquid fuels are made contains a diverse mixture of hydrocarbons and other compounds which vary widely in molecular weight and therefore boil over a wide range. For example, crude oils are known in which 30 to 60% or more of the total volume of oil is composed of compounds boiling at temperatures above 343.degree. C. (650.degree. F.). Among these are crudes in which about 10% to about 30% or more of the total volume consists of compounds so heavy in molecular weight that they boil above 552.degree. C. (1025.degree. F.) or at least will not boil below 552.degree. C. (1025.degree. F.) at atmospheric pressure.
Because these high boiling components of crude oil boiling above 343.degree. C. (650.degree. F.) are unsuitable for inclusion in gasoline and some higher boiling liquid hydrocarbon fuels, the petroleum refining industry has developed processes for separating and/or breaking the molecules of the high molecular weight, high boiling compounds into smaller molecules which do boil over an appropriate boiling range. The cracking process which is most widely used for this purpose is known as fluid catalytic cracking (FCC). Although the FCC process has reached a highly advanced state, and many modified forms and variations have been developed, their unifying factor is that a restricted boiling range hydrocarbon feedstock is caused to be cracked at an elevated temperature in contact with a cracking catalyst that is suspended in the feedstock under cracking conditions in a temperature range of 510.degree. C. to 593.degree. C. (950.degree. to 1100.degree. F.). Upon attainment of a desired degree of molecular weight and boiling point reduction, the catalyst is separated from the desired catalytic conversion products.
Crude oils in the natural state contain a variety of materials which tend to have quite troublesome effects on FCC processes, and only a portion of these troublesome materials can be economically removed from the crude oil. Among these troublesome materials are coke precursors (such as nickel, vanadium, iron, copper, etc.), heavy metals (such as nickel vanadium, iron, copper, etc.), lighter metals (such as sodium potassium, etc.), sulfur, nitrogen and others. Certain of these, such as the lighter metals, can be economically removed by desalting operations, which are part of the normal precedure for pretreating crude oil for fluid catalytic cracking. Other materials, such as coke precursors, asphaltenes and the like, tend to break down into coke during the cracking operation, which coke deposits on the catalyst, impairing contact between hydrocarbon feedstock and the catalyst, and generally reducing its potency or activity level. The heavy metals transfer almost quantitatively from the feedstock to the catalyst surface.
If the catalyst is reused again and again for processing additional feedstock, which is usually the case, heavy metals in the feedstock can accumulate on the catalyst to the point that they unfavorably alter the composition of the catalyst and/or the nature of its effect upon the feedstock. For example, vanadium pentoxide tends to form fluxes with certain components of commonly used FCC catalysts, lowering the melting point of portions of the catalyst particles sufficiently so that they begin to sinter and become ineffective cracking catalysts. Accumulations of vanadium and other heavy materials, especially nickel, also "poison" the catalyst. They tend in varying degrees to promote excessive dehydrogenation and aromatic condensation, resulting in excessive production of carbon and gases with consequent impairment of liquid fuel yield. An oil feed such as crude, or a crude residual fraction or other heavy oil fraction that is particularly abundant in nickel and vanadium or other metals exhibiting undesired behavior, containing relatively large quantities of coke forming materials comprising Conradson carbon, is referred to herein as carbo-metallic oil, and represents a particular challenge to the petroleum refiner.
Several proposals for upgrading bottom of the crude barrel or high boiling portions thereof involve treating a heavy oil feed to remove the metal and Conradson carbon contributing materials therefrom prior to catalytic cracking, such as by hydrotreating, solvent extraction, thermal conversion as by visbreaking and complexing with Friedel-Crafts catalysts, but these techniques have been criticized as unjustified economically. Another proposal employes a combination cracking process having "dirty oil" and "clean oil" units such as Becker U.S. Pat. No. 2,378,531 and Bartholic U.S. Pat. No. 4,263,128 and others. Still another proposal blends residual oil with gas oil and controls the quantity of residual oil in the mixture in relation to the equilibrium flash vaporization temperature at the bottom of the riser type cracking unit employed in the process. Still another proposal subjects the feed to a mild preliminary hydrocracking or hydrotreating operation before it is introduced into the cracking unit. It has also been suggested to contact a carbo-metallic oil such as reduced crude with hot taconite pellets to produce gasoline. This is a small sampling of the many proposals which have appeared in the patent literature and technical reports.